Peroxide curable fluoroelastomer compositions and articles made therefrom

ABSTRACT

Peroxide curable fluoroelastomer compositions and articles made therefrom are disclosed. The compositions contain 5 to 45 weight percent, based on total weight of all fluoroelastomers present in the composition, of a blend of dynamically cured fluoroelastomer particles in an uncured fluoroelastomer. The compositions are unfilled, but have tensile properties adequate for many end use applications and are particularly useful as seals in semiconductor manufacturing equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/068,128 filed Mar. 4, 2008.

FIELD OF THE INVENTION

This invention relates to peroxide curable fluoroelastomer compositionsthat are substantially free of filler and to cured articles madetherefrom wherein said fluoroelastomer compositions contain a peroxidecurable fluoroelastomer and an amount of gelled fluoroelastomer.

BACKGROUND OF THE INVENTION

Fluoroelastomers having excellent heat resistance, oil resistance, andchemical resistance have been used widely for sealing materials,containers and hoses. Examples of fluoroelastomers include copolymerscomprising units of vinylidene fluoride (VF₂) and units of at least oneother copolymerizable fluorine-containing monomer such ashexafluoropropylene (HFP), tetrafluoroethylene (TFE),chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a fluorovinylether such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examplesof PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinylether) and perfluoro(propyl vinyl ether).

In order to develop the physical properties necessary for most end useapplications, fluoroelastomers must be crosslinked (i.e. cured). Apreferred curing system for many end uses is the combination of anorganic peroxide and a multifunctional unsaturated coagent. The coagentforms crosslinks by reacting with bromine or iodine atom cure sites onthe fluoroelastomer polymer chain. A preferred cure site is an iodineatom bonded to a carbon atom on the fluoroelastomer chain.

Typical peroxide curable fluoroelastomer compositions also containfillers such as carbon black, SiO₂, TiO₂, BaSO₄, Al₂O₃, etc. Fillersgenerally improve the tensile properties of the cured fluoroelastomer.In the case of peroxide curable fluoroelastomer compositions, fillersalso improve the processability of the compositions on rubbermanufacturing equipment such as mills and extruders where slippage mayoccur in the absence of filler, and in compression molding where moldfouling can occur without filler present in the compositions. However,fillers may also be an undesirable source of contamination by metals orparticles in some end use applications such as in semiconductormanufacturing equipment where elastomer articles contact harshenvironments such as plasmas.

Unfilled peroxide curable fluoroelastomer compositions are known in theart, but they are typically difficult to manufacture and have lowtensile properties, making them unsuitable for some end useapplications.

Curable compositions of fluoroelastomers that contain particles ofdynamically cured fluoroelastomer particles (also referred to as gel)and filler have been previously disclosed, e.g. U.S. Pat. No. 4,708,988.The presence of gel appears to decrease the tensile strength of theperoxide cured compositions.

SUMMARY OF THE INVENTION

The present invention provides an unfilled curable fluoroelastomercomposition and cured article made therefrom that has adequate tensilestrength for many end use applications, reduced metal content and thatresults in reduced particles when exposed to reactive plasma.

Accordingly, one aspect of the present invention is a curablefluoroelastomer composition comprising:

A) 55 to 95 weight percent, based on total weight of allfluoroelastomers in said composition, of a first curable fluoroelastomercomprising copolymerized units of i) vinylidene fluoride and ii) atleast one other fluoromonomer, said first fluoroelastomer having curesites selected from the group consisting of iodine atoms, bromine atoms,and the combination of iodine and bromine atoms;

B) 5 to 45 weight percent, based on total weight of all fluoroelastomersin said composition, of a blend comprising i) 55 to 95 weight percent,based on total weight of said blend, of cured particles of a secondfluoroelastomer comprising copolymerized units of a) vinylidene fluorideand b) at least one other fluoromonomer, said second fluoroelastomerhaving cure sites selected from the group consisting of iodine atoms,bromine atoms, and the combination of iodine and bromine atoms; and ii)5 to 45 weight percent, based on total weight of said blend, of anuncured third fluoroelastomer comprising copolymerized units of a)vinylidene fluoride and b) at least one other fluoromonomer, said thirdfluoroelastomer having no bromine or iodine cure sites;

C) less than 1 weight percent, based on total weight of said curablecomposition, of filler;

D) 0.1 to 6 parts by weight organic peroxide per hundred parts by weightof said first curable fluoroelastomer; and

E) 0.1 to 10 parts by weight multifunctional unsaturated coagent perhundred parts by weight of said first curable fluoroelastomer.

Another aspect of the present invention is a cured article prepared fromthe above described curable fluoroelastomer composition.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to unfilled peroxide curablefluoroelastomer compositions and to cured articles prepared therefrom.By “unfilled” is meant that the composition contains less than 1(preferably less than 0.5, most preferably 0) weight percent fillerbased on the total weight of the curable fluoroelastomer composition. By“filler” is meant any filler commonly employed in the rubber industrysuch as carbon black, silica, titanium dioxide, alumina, barium sulfate,silicates of magnesium, calcium or aluminum, calcium carbonate, PTFEmicropowders, etc. In some end use applications, such as semiconductormanufacturing environments where the cured articles are exposed toreactive plasma, the unfilled cured articles of the invention releaseless contamination (e.g. metals and particles) than do filled curedarticles of similar composition. By “reactive plasma” is meant plasmathat reacts with fluoroelastomer; such plasmas include, O₂, CF₄, NF₃,etc.

Preferably the curable compositions and cured articles of the inventionare substantially free of metal oxides and hydroxides (e.g. MgO,Ca(OH)₂, PbO, ZnO, etc.) which, when present, can also be an undesirablesource of metal contamination in some end use environments. By“substantially free” is meant less than 1 (preferably less than 0.5,most preferably 0) weight percent metal oxide or metal hydroxide basedon the total weight of the fluoroelastomer composition.

A curable composition of the invention comprises A) 5 to 45 weightpercent, based on the total weight of all fluoroelastomers present inthe composition, of a curable first fluoroelastomer. In order to improveprocessability (e.g. mixing and extrusion) of the curable compositionsof the invention and to improve the tensile strength of cured articlesmade therefrom, the curable composition of the invention furthercomprises B) 5 to 45 (preferably 20 to 30) weight percent, based ontotal weight of all the fluoroelastomers present in the composition, ofa blend of i) 55 to 95 weight percent, based on total weight of saidblend, of dynamically cured particles (i.e. gel) of a secondfluoroelastomer dispersed in ii) 5 to 45 weight percent, based on totalweight of the blend, of an uncured third fluoroelastomer. The first andsecond fluoroelastomers are curable by organic peroxide (i.e. theycontain bromine, iodine or both bromine and iodine cure sites). Thefirst and second fluoroelastomers may be the same or of differentcomposition. The third fluoroelastomer does not contain such cure sitesand is not curable by peroxide

The blend of dynamically cured second fluoroelastomer particlesdispersed in an uncured third fluoroelastomer is made by introducinguncured second fluoroelastomer, third fluoroelastomer, 1 to 5 phrorganic peroxide and 1.5 to 10 multifunctional unsaturated coagent intoan internal mixer or an extruder at a temperature above thedecomposition temperature of the peroxide. The second fluoroelastomer isthen dynamically cured in the mixer to form gel particles dispersed inthe third uncured elastomer.

Curable compositions of the invention further comprise 0.1 to 6(preferably 1.5 to 3) parts by weight organic peroxide per hundred partsby weight of said curable first fluoroelastomer, and 0.1 to 10(preferably 1.5 to 3) parts by weight multifunctional unsaturatedcoagent per hundred parts by weight of said curable firstfluoroelastomer.

The fluoroelastomers employed in this invention comprise copolymerizedunits of vinylidene fluoride (VF₂) and one or more additionalfluoromonomers (not VF₂) such as those selected from the groupconsisting of fluorine-containing olefins, fluorine-containing ethersand mixtures thereof.

According to the present invention, fluorine-containing olefinscopolymerizable with vinylidene fluoride include, but are not limited tohexafluoropropylene (HFP), tetrafluoroethylene (TFE),1,2,3,3,3-pentafluoropropene (1-HPFP), chlorotrifluoroethylene (CTFE)and vinyl fluoride.

Fluorine-containing ethers that may be employed in the fluoroelastomersinclude, but are not limited to perfluoro(alkyl vinyl ethers),perfluoro(alkyl alkenyl ethers) and perfluoro(alkoxy alkenylethers).

Perfluoro(alkyl vinyl ethers) (PAVE) suitable for use as monomersinclude those of the formula

CF₂═CFO(R_(f′)O)_(n)(R_(f″)O)_(m)R_(f)   (I)

where R_(f′) and R_(f″) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl ethers) includes compositionsof the formula

CF₂═CFO(CF₂CFXO)_(n)R_(f)   (II)

where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl group of1-6 carbon atoms. A most preferred class of perfluoro(alkyl vinylethers) includes those ethers wherein n is 0 or 1 and R_(f) contains 1-3carbon atoms. Examples of such perfluorinated ethers includeperfluoro(methyl vinyl ether) (PMVE) and perfluoro(propyl vinyl ether)(PPVE). Other useful monomers include compounds of the formula

CF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)R_(f)   (III)

where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1,n=0-5, and Z=F or CF₃. Preferred members of this class are those inwhich R_(f) is C₃F₇, m=0, and n=1.

Additional perfluoro(alkyl vinyl ether) monomers include compounds ofthe formula

CF₂═CFO[(CF₂CF{CF₃}O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)   (IV)

where m and n independently=0-10, p=0-3, and x=1-5. Preferred members ofthis class include compounds where n=0-1, m=0-1, and x=1.

Other examples of useful perfluoro(alkyl vinyl ethers) include

CF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)   (V)

where n=1-5, m=1-3, and where, preferably, n=1.

Perfluoro(alkyl alkenyl ethers) suitable for use as monomers includethose of the formula VI

R_(f)O(CF₂)_(n)CF═CF₂   (VI)

where R_(f) is a perfluorinated linear or branched aliphatic groupcontaining 1-20, preferably 1-10, and most preferably 1-4 carbon atomsand n is an integer between 1 and 4. Specific examples include, but arenot limited to perfluoro(propoxyallyl ether) andperfluoro(propoxybutenyl ether).

Perfluoro(alkoxy alkenyl ethers) differ from perfluoro(alkyl alkenylethers) in that R_(f) in formula VI contains at least one oxygen atom inthe aliphatic chain. A specific example includes, but is not limited toperfluoro(methoxyethoxyallyl ether).

Peroxide curable fluoroelastomers that may be employed as the first andsecond fluoroelastomers of the present invention also contain cure sitesof bromine atoms, iodine atoms or both. The cure sites may be along thefluoroelastomer chain (i.e. due to copolymerized units of cure sitemonomer containing bromine or iodine atoms), at chain ends (i.e. due topolymerization in the presence of an iodine or bromine atom-containingchain transfer agent), or both along fluoroelastomer chains and at chainends.

Brominated cure site monomers may contain other halogens, preferablyfluorine. Examples of brominated olefin cure site monomers areCF₂═CFOCF₂CF₂CF₂OCF₂CF₂Br; bromotrifluoroethylene;4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB); and others such as vinylbromide, 1-bromo-2,2-difluoroethylene; perfluoroallyl bromide;4-bromo-1,1,2-trifluorobutene-1; 4-bromo-1,1,3,3,4,4,-hexafluorobutene;4-bromo-3-chloro-1,1,3,4,4-pentafluorobutene;6-bromo-5,5,6,6-tetrafluorohexene; 4-bromoperfluorobutene-1 and3,3-difluoroallyl bromide. Brominated vinyl ether cure site monomersuseful in the invention include 2-bromo-perfluoroethyl perfluorovinylether and fluorinated compounds of the class CF₂Br—R_(f)—O—CF═CF₂ (R_(f)is a perfluoroalkylene group), such as CF₂BrCF₂O—CF═CF₂, and fluorovinylethers of the class ROCF═CFBr or ROCBr═CF₂ (where R is a lower alkylgroup or fluoroalkyl group) such as CH₃OCF═CFBr or CF₃CH₂OCF═CFBr.

Suitable iodinated cure site monomers include iodinated olefins of theformula: CHR═CH-Z-CH₂CHR—I, wherein R is —H or —CH₃; Z is aC₁-C₁₈(per)fluoroalkylene radical, linear or branched, optionallycontaining one or more ether oxygen atoms, or a(per)fluoropolyoxyalkylene radical as disclosed in U.S. Pat. No.5,674,959. Other examples of useful iodinated cure site monomers areunsaturated ethers of the formula: I(CH₂CF₂CF₂)_(n)OCF═CF₂ andICH₂CF₂O[CF(CF₃)CF₂O]_(n)CF═CF₂, and the like, wherein n=1-3, such asdisclosed in U.S. Pat. No. 5,717,036. In addition, suitable iodinatedcure site monomers including iodoethylene,4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB);3-chloro-4-iodo-3,4,4-trifluorobutene;2-iodo-1,1,2,2-tetrafluoro-1-(vinyloxy)ethane;2-iodo-1-(perfluorovinyloxy)-1,1,-2,2-tetrafluoroethylene;1,1,2,3,3,3-hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethylvinyl ether; 3,3,4,5,5,5-hexafluoro-4-iodopentene; andiodotrifluoroethylene are disclosed in U.S. Pat. No. 4,694,045. Allyliodide and 2-iodo-perfluoroethyl perfluorovinyl ether are also usefulcure site monomers.

Units of cure site monomer, when present in the first or secondfluoroelastomers employed in this invention, are typically present at alevel of 0.05-10 wt. % (based on the total weight of fluoroelastomer),preferably 0.05-5 wt. % and most preferably between 0.05 and 3 wt. %.

Additionally or in place of copolymerized cure site monomer,iodine-containing endgroups, bromine-containing endgroups or mixturesthereof may optionally be present at one or both of the fluoroelastomerpolymer chain ends as a result of the use of chain transfer or molecularweight regulating agents during preparation of the fluoroelastomers. Theamount of chain transfer agent, when employed, is calculated to resultin an iodine or bromine level in the first or second fluoroelastomer inthe range of 0.005-5 wt. %, preferably 0.05-3 wt. %.

Examples of chain transfer agents include iodine-containing compoundsthat result in incorporation of bound iodine at one or both ends of thepolymer molecules. Methylene iodide; 1,4-diiodoperfluoro-n-butane; and1,6-diiodo-3,3,4,4,tetrafluorohexane are representative of such agents.Other iodinated chain transfer agents include1,3-diiodoperfluoropropane; 1,6-diiodoperfluorohexane;1,3-diiodo-2-chloroperfluoropropane;1,2-di(iododifluoromethyl)-perfluorocyclobutane;monoiodoperfluoroethane; monoiodoperfluorobutane;2-iodo-1-hydroperfluoroethane, etc. Also included are the cyano-iodinechain transfer agents disclosed European Patent 0868447A1. Particularlypreferred are diiodinated chain transfer agents.

Examples of brominated chain transfer agents include1-bromo-2-iodoperfluoroethane; 1-bromo-3-iodoperfluoropropane;1-iodo-2-bromo-1,1-difluoroethane and others such as disclosed in U.S.Pat. No. 5,151,492.

Two preferred fluoroelastomers that may be employed in this inventioncomprise copolymerized units of A) vinylidene fluoride,hexafluoropropylene and, optionally, tetrafluoroethylene and B)vinylidene fluoride, perfluoro(methyl vinyl ether) andtetrafluoroethylene. In the case of the first and secondfluoroelastomers, each also contain cure sites of bromine atoms, iodineatoms, or both bromine and iodine atoms. The third fluoroelastomeremployed in the compositions of the invention does not contain reactivesites suitable for a peroxide cure, e.g. no bromine or iodine atoms, andis thus not peroxide curable.

Organic peroxides suitable to make the dynamically cured fluoroelastomergel particles or to cure the overall curable fluoroelastomer compositioninclude, but are not limited to1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane;1,1-bis(t-butylperoxy)cyclohexane; 2,2-bis(t-butylperoxy)octane;n-butyl-4,4-bis(t-butylperoxy)valerate; 2,2-bis(t-butylperoxy)butane;2,5-dimethylhexane-2,5-dihydroxyperoxide; di-t-butyl peroxide;t-butylcumyl peroxide; dicumyl peroxide; alpha,alpha′-bis(t-butylperoxy-m-isopropyl)benzene;2,5-dimethyl-2,5-di(t-butylperoxy)hexane;2,5-dimethyl-2,5-di(t-butylperoxy)hexene-3; benzoyl peroxide,t-butylperoxybenzene; 2,5-dimethyl-2,5-di(benzoylperoxy)-hexane;t-butylperoxymaleic acid; and t-butylperoxyisopropylcarbonate. Preferredexamples of organic peroxides include2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, and alpha,alpha′-bis(t-butylperoxy-m-isopropyl)benzene.

Multifunctional unsaturated coagents that may be employed to make thedynamically cured fluoroelastomer gel particles or to cure the overallcurable fluoroelastomer composition are polyfunctional unsaturatedcompounds such as triallyl cyanurate, trimethacryl isocyanurate,triallyl isocyanurate, trimethallyl isocyanurate, triacryl formal,triallyl trimellitate, N,N′-m-phenylene bismaleimide, diallyl phthalate,tetraallylterephthalamide, tri(diallylamine)-s-triazine, triallylphosphite, bis-olefins and N,N-diallylacrylamide.

Curable compositions of the invention are made by combining i) the firstfluoroelastomer, ii) blend of cured second fluoroelastomer particles inthird fluoroelastomer, iii) organic peroxide, and iv) coagent by meansof an internal mixer, extruder, or rubber mill, operating at atemperature below the temperature where the organic peroxide decomposes.The resulting composition may then be shaped (e.g. molded or extruded)and cured to form cured fluoroelastomer articles. Curing typically takesplace at about 150°-200° C. for 1 to 60 minutes. Conventional rubbercuring presses, molds, extruders, and the like provided with suitableheating and curing means can be used. Also, for optimum physicalproperties and dimensional stability, it is preferred to carry out apost curing operation wherein the molded or extruded fluororubbercomponent is heated in an oven or the like for an additional period ofabout 1-48 hours, typically from about 180°-275° C., generally in an airatmosphere.

Cured articles (e.g. seals, hoses, diaphragms, etc.) of the inventionare useful in may end use applications, especially in applications wherethey will be exposed to reactive plasma such as seals in semiconductormanufacturing equipment.

EXAMPLES Test Methods Tensile Properties

The following physical property parameters were recorded; test methodsare in parentheses:

T_(b): tensile strength, MPa (ASTM D412-92)

E_(b): elongation at break, % (ASTM D412-92)

M100: modulus at 100% elongation, MPa (ASTM D412-92)

Hardness, Shore A (ASTM D412-92)

Compression Set B (ASTM D395)

Mooney Scorch (ASTM D1646)

Curing Characteristics

Moving Disc Rheometer (MDR) (ASTM D5289)

The invention is further illustrated by, but is not limited to, thefollowing examples.

Example 1

A blend of gel particles dispersed in an uncured fluoroelastomer wasmade by combining i) 80 parts by weight of a peroxide curablefluoroelastomer (FKM1) comprising copolymerized units of vinylidenefluoride (VF₂), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE)and containing iodine cure sites, ii) 20 parts by weight of afluoroelastomer (FKM2) comprising copolymerized units of VF₂ and HFP (noiodine or bromine atom cure sites), iii) 1.75 parts by weight triallylisocyanurate coagent (Diak 7 available from DuPont PerformanceElastomers) and iv) 1.75 parts by weight organic peroxide (Varox® DBPHavailable from available from R.T. Vanderbilt) in a Banbury® internalmixer at 160° C. for 45 minutes.

A curable composition of the invention that contained the gel blendprepared above (Sample 1) and a similar control composition without gelparticles (Control A) were mixed on a two roll rubber mill at roomtemperature so as not to decompose the organic peroxide and causepremature curing. The formulations are shown in Table I. Amounts areparts by weight.

Curing characteristics and physical properties were measured accordingto the Test methods. Results are also shown in Table I. Sample 1 of theinvention (contained gel) had a better (i.e. higher) tensile strength(Tb) and better (i.e. lower) compression set than that of the controlcomposition (no gel). Sample 1 also cured faster (i.e. lower t′90) thandid the control composition. In addition, Sample 1 had a higherviscosity (ML from the MDR test) than did Control A which results inimproved backpressure in an O-ring compression molding process.

TABLE I Control A Sample 1 Ingredient FKM1 100 75 Diak 7 1.5 1.5 Varox ®DBPH liquid 1.75 1.5 Gel Blend 0 25 Mooney Scorch @121° C. 2 pt. rise(minutes) 29.7 18.7 MDR @165° C., 0.5° arc, 12 minutes ML (dN · m) 1.11.7 MH (dN · m) 10.9 11.8 Ts2 (minutes) 1.6 1.0 T′90 (minutes) 6.6 4.1Physical Properties @25° C., 214 O-rings, 7 minute press cure 162° C., 4hour post cure @160° C. M100, MPa 1.1 1.1 Tb, MPa 3.9 7.6 Eb, % 376 383Compression Set of 214 O- rings cured as above 70 hours, 200° C. 61 45

Example 2

A blend of gel particles dispersed in an uncured fluoroelastomer wasmade by combining i) 80 parts by weight of a peroxide curablefluoroelastomer (FKM1) comprising copolymerized units of vinylidenefluoride (VF₂), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE)and containing iodine cure sites, ii) 20 parts by weight of afluoroelastomer (FKM2) comprising copolymerized units of VF₂ and HFP (noiodine or bromine atom cure sites), iii) 9 parts by weight triallylisocyanurate coagent (Diak 7 available from DuPont PerformanceElastomers) and iv) 3 parts by weight organic peroxide (Varox® DBPHavailable from available from R.T. Vanderbilt) in a Banbury® internalmixer at 160° C. for 45 minutes.

A curable composition of the invention that contained the gel blendprepared above (Sample 2) and a similar control composition without gelparticles (Control B) were mixed on a two roll rubber mill at roomtemperature so as not to decompose the organic peroxide and causepremature curing. The formulations are shown in Table II. Amounts areparts by weight. It only took 15 minutes to mix the composition of theinvention (Sample 2), whereas it took 45 minutes to mix the controlcomposition without gel (Control B).

Curing characteristics and physical properties were measured accordingto the Test methods. Results are also shown in Table II. Sample 2 of theinvention (contained gel) had a better (i.e. higher) tensile strength(Tb) than that of the control composition (no gel). Sample 2 also curedfaster (i.e. lower t′90) than did the control composition.

TABLE II Control B Sample 2 Ingredient FKM1 97.75 77.75 Diak 7 3 3Varox ® DBPH liquid 1.25 1.25 Gel Blend 0 20 Mooney Scorch @121° C. 2pt. rise (minutes) >30 13.8 MDR @165° C., 0.5° arc, 12 minutes ML (dN ·m) 1.0 1.8 MH (dN · m) 13.5 14.5 Ts2 (minutes) 1.2 0.8 T′90 (minutes)6.3 2.8 Physical Properties @25° C., 214 O-rings, 10 minute press cure177° C., 4 hour post cure @160° C. M100, MPa 1.1 1.3 Tb, MPa 7.2 10.2Eb, % 374 336 Compression Set of 214 O- rings cured as above 70 hours,200° C. 29 30

1. A curable fluoroelastomer composition comprising: A) 55 to 95 weightpercent, based on total weight of all fluoroelastomers in saidcomposition, of a first curable fluoroelastomer comprising copolymerizedunits of i) vinylidene fluoride and ii) at least one otherfluoromonomer, said first fluoroelastomer having cure sites selectedfrom the group consisting of iodine atoms, bromine atoms, and thecombination of iodine and bromine atoms; B) 5 to 45 weight percent,based on total weight of all fluoroelastomers in said composition, of ablend comprising i) 55 to 95 weight percent, based on total weight ofsaid blend, of cured particles of a second fluoroelastomer comprisingcopolymerized units of a) vinylidene fluoride and b) at least one otherfluoromonomer, said second fluoroelastomer having cure sites selectedfrom the group consisting of iodine atoms, bromine atoms, and thecombination of iodine and bromine atoms; and ii) 5 to 45 weight percent,based on total weight of said blend, of an uncured third fluoroelastomercomprising copolymerized units of a) vinylidene fluoride and b) at leastone other fluoromonomer, said third fluoroelastomer having no bromine oriodine cure sites; C) less than 1 weight percent, based on total weightof said curable composition, of filler; D) 0.1 to 6 parts by weightorganic peroxide per hundred parts by weight of said first curablefluoroelastomer; and E) 0.1 to 10 parts by weight multifunctionalunsaturated coagent per hundred parts by weight of said first curablefluoroelastomer.
 2. A curable fluoroelastomer composition of claim 1wherein said first fluoroelastomer comprises copolymerized unitsselected from the groups consisting of i) vinylidene fluoride,hexafluoropropylene and optionally tetrafluoroethylene, and ii)vinylidene fluoride, perfluoro(methyl vinyl ether) andtetrafluoroethylene.
 3. A curable fluoroelastomer composition of claim 2wherein said first fluoroelastomer further comprises copolymerized unitsof a cure site monomer selected from the group consisting ofiodine-containing cure site monomers, bromine-containing cure sitemonomers, and mixtures thereof.
 4. A curable fluoroelastomer compositionof claim 2 wherein said first fluoroelastomer further comprises curesites selected from the group consisting of iodine-containing endgroupsand bromine-containing endgroups.
 5. A curable fluoroelastomercomposition of claim 1 wherein said second fluoroelastomer comprisescopolymerized units selected from the groups consisting of i) vinylidenefluoride, hexafluoropropylene and optionally tetrafluoroethylene, andii) vinylidene fluoride, perfluoro(methyl vinyl ether) andtetrafluoroethylene.
 6. A curable fluoroelastomer composition of claim 5wherein said second fluoroelastomer further comprises copolymerizedunits of a cure site monomer selected from the group consisting ofiodine-containing cure site monomers, bromine-containing cure sitemonomers, and mixtures thereof.
 7. A curable fluoroelastomer compositionof claim 5 wherein said second fluoroelastomer further comprises curesites selected from the group consisting of iodine-containing endgroupsand bromine-containing endgroups.
 8. A curable fluoroelastomercomposition of claim 1 wherein said first fluoroelastomer and saidsecond fluoroelastomer are the same fluoroelastomer.
 9. A curablefluoroelastomer composition of claim 1 wherein said thirdfluoroelastomer comprises copolymerized units selected from the groupconsisting of i) vinylidene fluoride, hexafluoropropylene and optionallytetrafluoroethylene, and ii) vinylidene fluoride, perfluoro(methyl vinylether) and tetrafluoroethylene.
 10. A cured article made from thecurable fluoroelastomer composition of claim 1.